The invention relates to a method and apparatus for reducing electrical noise and electromagnetic radiation in electronic packages and, more specifically, for decoupling ground and power conductors in electronic packages.
It is well known that electronic packages, such as printed circuit cards and modules, as well as chips include a power distribution system to distribute power to operate electronic circuits connected thereto. When an electronic circuit performs a switching operation, current pulses are drawn from the power distribution system, such as the power and ground planes in a printed circuit card or the power and ground wiring in a chip. As the power distribution system presents a finite impedance to the electronic circuit, the switching operation results in current and voltage transients. Such voltage fluctuations are typically referred as Delta-I noise.
Voltage transients can have a number of adverse effects. For instance, the voltage transient may cause voltage supplied to the switching circuit to drop. As a result, the output of the switching circuit may be outside the normal operating range or specification. The voltage transient may also influence other electronic circuits in a similar manner, thus causing them to malfunction or operate in an inappropriate manner. In addition, voltage transients may lead to undesirable electromagnetic radiation from the electronic package, which can interfere with radio communications. These problems are further complicated if resonances are excited.
One common approach to minimizing voltage disturbances in power distribution systems is to place numerous decoupling capacitors between the power and ground conductors at different locations. The capacitors supply charge to stabilize the voltage. Such an approach is effective at low clock frequencies, where the parasitic inductance of the capacitors is not significant and where the system size is much less than the wavelength corresponding to the highest frequencies contained in the clock signals.
However, as electronic circuits become more complex and operate at higher clock frequencies, the power distribution system acts as a transmission medium or line which allows propagation of traveling waves and reflections from edges and other discontinuities of the electronic package. For example, such standing waves are known to exist at frequencies where multiples of a half wavelength equal the electrical length of the printed circuit card. Decoupling capacitors and their connecting leads have a significant parasitic series inductance, and can result in additional resonances.
There is a need to provide a better method and apparatus for reducing noise between power and ground conductors of electronic packages and radiated electromagnetic interference. There is also a need to provide such a method utilizing minimum hardware and at a minimum cost.
Accordingly, it is an object of the present invention to provide a method and apparatus for reducing electrical noise and electromagnetic radiation in electronic packages, such as printed circuit cards, modules and the like, as well as chips.
It is a further object of the present invention to provide a method and apparatus which reduces electrical noise and electromagnetic radiation in electronic packages employing high clock frequencies.
It is also an object of the present invention to provide a method and apparatus which employ a series resistor-capacitor combination to decouple electronic packages.
The present invention is a method and apparatus for electrically decoupling conductors used to distribute power and ground potentials in electronic packages and chips. Specifically, the present invention utilizes Absorbing Plane Terminators (APTs), which are connected between ground and power conductors of the electronic package. An APT includes a resistor with a value preferably chosen to match the approximate characteristic impedance of the plane structures, and a decoupling capacitor connected in series with the resistor. The APT reduces electronic noise and electromagnetic radiation over a wide range of clock operating frequencies. The APT also dampens resonances caused by the parasitic inductance of the decoupling capacitor.
It is preferred that a plurality of APTs are placed along edges (e.g. the periphery) and at other discontinuities of the conductors and at uniform spaced-apart intervals. The APTs absorb time varying signals, such as traveling waves, that propagate toward the edges. As a result, the APTs substantially eliminate reflection signals along the edges and prevents resonances between the conductors.